What organism is the common ancestor to plants?

Many Protists share traits with plants


  • Dinoflagellates - unicellular aquatic
    • some are photosynthetic
    • but haploid dominate life cycle


  • Brown Algae - large multicellular aquatic
    • AoG life cycle
    • photosynthetic but use chlorophyll c


  • Diatoms - unicellular aquatic
    • all are photosynthetic
    • but diploid dominated life cycle





Dinoflagellates
Brown Algae
Diatoms


Not ancestral to plants!


Secondary endosymbiosis
for chloroplasts

The big picture: Plants evolved from green algae


  • Cyanobacteria & protists made landfall ~1.2 bya
    • plants, fungi and animals ~500 mya
    • first forests 385 mya


  • Plants evolved from green algae
    • several key ‘shared derived traits’


  • Plants now support all other life on land
    • oxygen to breath
    • food to eat
    • new habitats

Green Algae (genus = Oedogonium)


  • Sister group to land plants (embryophytes)
    • all are photosynthetic
    • marine and freshwater
    • single and multi-cellular
    • broad, thick filaments (not stems)
    • some species have A.o.G
    • evolved around ~750mya


  • Key reproductive features:
    • Oogonia = egg containing cell
    • Antheridium = sperm containing cell
    • these ‘houses’ for egg and sperm are single-celled protections for drier environments

Molecular evidence points at charophytes as plant ancestors

Many shared gene families with plants, now that we have the chara genome

Charophytes (freshwater green algae)

  • Freshwater species


  • Transition from water to land starts with Charophytes
    • freshwater habitats may have dried
    • ancestral species partly lived out of water


  • More shared traits with land plants: know a few!
    • circular protein rings in plasma membrane
      • make cellulose fibers in cell wall
    • swimming sperm with similar structure
    • phragmoplast (cell division/cell wall formation)
    • ROS genes to eliminate free oxygen radicals

Living on Land: The Wild Wild West

What are the pros and cons of living on land?

Land plants are a monophyletic group

Shared derived traits define land plant evolution

Multi-cellular, Dependent embryos (placental transfer)

Fancy botany word for plants = embryophytes (embryo retained on gametophyte)

Waxy cuticle and stomata

Multi-cellular Gametangia

Fancy botany term for the sperm and egg ‘houses’

Photosynthesis with unique pigments (chlorophyll a & b)

Chlorophyll a/b are specialized to absorb the intense direct solar radiation at specific wavelengths

Unique cell walls


  • Cells walls made of cellulose
    • not unique to plants


  • Pectin to fortify cell walls
    • needed by plants for mechanical strength
    • also used as cement to hold adjacent cells together
    • ‘pectinase’ is an enzyme that degrades pectin in ripening fruit!


  • Produce cells walls in unique way
    • at end of mitosis

Alternation of generations

Can you draw me yet?

Alternation of generations - Homosporous

Homospory - one type of spore, that germinates into a bisexual gametophyte

First plant group: non-vascular bryophytes (mosses)


  • Mosses, hornworts and liverworts
    • Fossils of bryophyte spores ~470mya


  • Non-vascular; ground hugging carpets
    • bodies to thin to support height growth


  • Have a rhizoid but not a root
    • anchors plant
    • does not uptake water


  • Very resistant spores

Bryophytes have a gametophyte dominated life cycle

Early plants, like moss, have sperm that swim from antheridia to archegonia